String based erase inhibit

ABSTRACT

A non-volatile memory device, described herein, comprises: a plurality of memory strings and at least one control circuit in communication with the non-volatile memory cell array. The at least one control circuit is configured to perform, for the plurality of memory strings, one erase-verify iteration in an erase operation including determining whether at least one memory string of the plurality of memory strings passes an erase-verify test. The at least one control circuit is configured to, if the at least one memory string passes the erase-verify test, inhibit the at least one memory string for erase including ramping up, to an erase voltage, of a voltage applied to a gate of a SGD transistor of the at least one memory string and to perform a next erase-verify iteration in the erase operation for remaining memory strings of the plurality of memory strings other than the at least one memory string.

FIELD

This application relates to non-volatile memory apparatuses and theoperation of non-volatile memory apparatuses.

BACKGROUND

This section provides background information related to the technologyassociated with the present disclosure and, as such, is not necessarilyprior art.

Semiconductor memory apparatuses have become more popular for use invarious electronic devices. For example, non-volatile semiconductormemory is used in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices.

A charge-storing material such as a floating gate or a charge-trappingmaterial can be used in such memory apparatuses to store a charge whichrepresents a data state. A charge-trapping material can be arrangedvertically in a three-dimensional (3D) stacked memory structure, orhorizontally in a two-dimensional (2D) memory structure. One example ofa 3D memory structure is the Bit Cost Scalable (BiCS) architecture whichcomprises a stack of alternating conductive and dielectric layers.

SUMMARY

This section provides a general summary of the present disclosure and isnot a comprehensive disclosure of its full scope or all of its featuresand advantages.

An object of the present disclosure is to provide a memory apparatus anda method of operation of the memory apparatus that address and overcomeshortcomings described herein.

Accordingly, it is an aspect of the present disclosure to a non-volatilememory device, comprising: a substrate; a non-volatile memory cell arraycarried by the substrate and comprising a plurality of memory strings,each memory string of the plurality of memory strings comprising aplurality of memory cells arranged between a select gate drain (SGD)transistor at a drain-side end of a memory string and a select gatesource (SGS) transistor at a source-side end of the memory string; andat least one control circuit in communication with the non-volatilememory cell array. The at least one control circuit: to perform, for theplurality of memory strings, one erase-verify iteration in an eraseoperation including determining whether at least one memory string ofthe plurality of memory strings passes an erase-verify test; and if theat least one memory string passes the erase-verify test: to inhibit theat least one memory string for erase including ramping up, to an erasevoltage, of a voltage applied to a gate of a SGD transistor of the atleast one memory string; and to perform a next erase-verify iteration inthe erase operation for remaining memory strings of the plurality ofmemory strings other than the at least one memory string.

Additionally, it is an aspect of the present disclosure to provide amethod of A method of operating a memory apparatus comprising aplurality of memory strings, each memory string of the plurality ofmemory strings comprising a plurality of memory cells arranged between aselect gate drain (SGD) transistor at a drain-side end of a memorystring and a select gate source (SGS) transistor at a source-side end ofthe memory string, the method comprising the steps of: performing, forthe plurality of memory strings, one erase-verify iteration in an eraseoperation including determining whether at least one memory string ofthe plurality of memory strings passes an erase-verify test; and if theat least one memory string passes the erase-verify test: inhibiting theat least one memory string for erase including ramping up, to an erasevoltage, of a voltage applied to a gate of a SGD transistor of the atleast one memory string; and performing a next erase-verify iteration inthe erase operation for remaining memory strings of the plurality ofmemory strings other than the at least one memory string.

Additionally, it is an aspect of the present disclosure to provide anapparatus comprising: means for determining whether at least one memorystring of a plurality of memory strings passes an erase-verify test of afirst erase-verify iteration in an erase operation; and means forramping up, to an erase voltage, of a voltage applied to a gate of a SGDtransistor of the at least one memory string if the at least one memorystring passes the erase-verify test; and means for performing a nexterase-verify iteration in the erase operation for remaining memorystrings of the plurality of memory strings other than the at least onememory string.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of example embodiments, reference will now bemade to the accompanying drawings in which:

FIG. 1A is a block diagram of an example memory device;

FIG. 1B is a block diagram of an example control circuit which comprisesa programming circuit, a counting circuit, and a determination circuit;

FIG. 2 depicts blocks of memory cells in an example two-dimensionalconfiguration of the memory array of FIG. 1 ;

FIG. 3A depicts a cross-sectional view of example floating gate memorycells in NAND strings;

FIG. 3B depicts a cross-sectional view of the structure of FIG. 3A alongline 329;

FIG. 4A depicts a cross-sectional view of example charge-trapping memorycells in NAND strings;

FIG. 4B depicts a cross-sectional view of the structure of FIG. 4A alongline 429;

FIG. 5A depicts an example block diagram of the sense block SB1 of FIG.1 ;

FIG. 5B depicts another example block diagram of the sense block SB1 ofFIG. 1 ;

FIG. 6A is a perspective view of a set of blocks in an examplethree-dimensional configuration of the memory array of FIG. 1 ;

FIG. 6B depicts an example cross-sectional view of a portion of one ofthe blocks of FIG. 6A;

FIG. 6C depicts a plot of memory hole diameter in the stack of FIG. 6B;

FIG. 6D depicts a close-up view of the region 622 of the stack of FIG.6B;

FIG. 7A depicts a top view of an example word line layer WLL0 of thestack of FIG. 6B;

FIG. 7B depicts a top view of an example top dielectric layer DL19 ofthe stack of FIG. 6B;

FIG. 8A depicts example NAND strings in the sub-blocks SBa-SBd of FIG.7A;

FIG. 8B depicts another example view of NAND strings in sub-blocks;

FIG. 8C depicts a top view of example word line layers of a stack;

FIG. 9 depicts the Vth distributions of memory cells in an exampleone-pass programming operation with four data states;

FIG. 10 depicts the Vth distributions of memory cells in an exampleone-pass programming operation with eight data states;

FIG. 11 depicts the Vth distributions of memory cells in an exampleone-pass programming operation with sixteen data states;

FIG. 12 is a flowchart of an example programming operation in a memorydevice;

FIGS. 13A and 13B depict the Vth distributions of memory cells;

FIG. 14A depicts threshold voltage distributions of an erased state andhigher data states;

FIG. 14B depicts a series of erase pulses and verify pulses in an eraseoperation;

FIG. 15 depicts a method of implementing a string-based erase inhibitscheme;

FIG. 16 an example sequence of the string-based erase inhibit scheme;

FIG. 17 is an example embodiment of a signal timing diagram ofstring-based erase inhibit scheme during an erase-verify iteration in anerase operation;

FIG. 18 provides an example embodiment of an expected waveform ofSGD_USEL.

DETAILED DESCRIPTION

In the following description, details are set forth to provide anunderstanding of the present disclosure. In some instances, certaincircuits, structures and techniques have not been described or shown indetail in order not to obscure the disclosure.

In general, the present disclosure relates to non-volatile memoryapparatuses of the type well-suited for use in many applications. Thenon-volatile memory apparatus and associated methods of forming of thisdisclosure will be described in conjunction with one or more exampleembodiments. However, the specific example embodiments disclosed aremerely provided to describe the inventive concepts, features, advantagesand objectives with sufficient clarity to permit those skilled in thisart to understand and practice the disclosure. Specifically, the exampleembodiments are provided so that this disclosure will be thorough, andwill fully convey the scope to those who are skilled in the art.Numerous specific details are set forth such as examples of specificcomponents, devices, and methods, to provide a thorough understanding ofembodiments of the present disclosure. It will be apparent to thoseskilled in the art that specific details need not be employed, thatexample embodiments may be embodied in many different forms and thatneither should be construed to limit the scope of the disclosure. Insome example embodiments, well-known processes, well-known devicestructures, and well-known technologies are not described in detail.

Various terms are used to refer to particular system components.Different companies may refer to a component by different names—thisdocument does not intend to distinguish between components that differin name but not function. In the following discussion and in the claims,the terms “including” and “comprising” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to . . . .” Also, the term “couple” or “couples” is intended tomean either an indirect or direct connection. Thus, if a first devicecouples to a second device, that connection may be through a directconnection or through an indirect connection via other devices andconnections.

Additionally, when a layer or element is referred to as being “on”another layer or substrate, in can be directly on the other layer ofsubstrate, or intervening layers may also be present. Further, it willbe understood that when a layer is referred to as being “under” anotherlayer, it can be directly under, and one or more intervening layers mayalso be present. Furthermore, when a layer is referred to as “between”two layers, it can be the only layer between the two layers, or one ormore intervening layers may also be present.

As described, non-volatile memory systems are a type of memory thatretains stored information without requiring an external power source.Non-volatile memory is widely used in various electronic devices and instand-alone memory devices. For example, non-volatile memory can befound in laptops, digital audio player, digital cameras, smart phones,video games, scientific instruments, industrial robots, medicalelectronics, solid-state drives, USB drives, memory cards, and the like.Non-volatile memory can be electronically programmed/reprogrammed anderased.

Examples of non-volatile memory systems include flash memory, such asNAND flash or NOR flash. NAND flash memory structures typically arrangemultiple memory cell transistors (e.g., floating-gate transistors orcharge trap transistors) in series with and between two select gates(e.g., a drain-side select gate and a source-side select gate). Thememory cell transistors in series and the select gates may be referredto as a NAND string. NAND flash memory may be scaled in order to reducecost per bit.

A 3D stacked non-volatile memory device can be arranged in multipleblocks, where typically an erase operation is performed one block at atime. An erase operation can include multiple erase-verify iterationswhich are performed until an erase-verify condition is met for theblock, at which point the erase operation ends. One approach is for theerase-verify condition to allow a predetermined number of fail bits.That is, the erase operation can be declared to be successful even if asmall number of memory cells have not reached the erase state. However,this approach does not inhibit fast-erasing memory cells fromover-erase. As a result, over-erase of some of the memory cells canoccur, causing serious degradation of the memory cells as excessiveholes are accumulated in the tunneling path.

However, unlike a 2D NAND structure, where a p-well substrate is commonfor all blocks, 3D stacked non-volatile memory devices have anindividual thin poly-silicon body for each NAND string channel, whosebias can be controlled by bit line (BL), source line (SL), drain-sideselect gate (SGD) and source-side select gate (SGS) voltages. In anormal erase operation, referred to as a two-sided erase, gate-induceddrain leakage (GIDL) currents are generated at both the SGD and SGStransistors. The BL and SL are biased at Verase (VERA), and SGD and SGSare biased at Vsg. In one approach, once all the memory cells associatedwith the same bit line pass an erase-verify test (e.g., reach the erasestate), the associated bit line voltage is reduced to Vsg+(0˜2V), sothat no GIDL current is generated at the next erase pulse at the bitline/drain side. Meanwhile, the source line voltage is also reduced toVsg+(0{tilde over ( )}2V) so that, for all channels, there will be noGIDL current generated at the source line side for all the followingerase pulses of the erase operation. Thus, erase inhibit is achieved forthe memory cells which pass the erase-verify test, while those that didnot pass are then erased by GIDL current generated at the bit line sideonly, in a one-sided erase. This avoids over erase of the cells whichreach the erase state relatively quickly.

To further help avoid over erase, embodiments described herein aredirected to a string-based erase inhibit. For example, during anerase-verify iteration of an erase operation, a memory string may beinhibited or unselected once the memory string passes the erase-verifytest. In some embodiments, a memory string may be inhibited byinitiating ramp up, to an erase voltage, of a voltage applied to a gateof a SGD transistor of the memory string before initiating ramp up, tothe erase voltage, of a voltage applied to a bit line connected to adrain-side end of the memory string. In some embodiments, a memorystring may be inhibited by initiating ramp up, to the erase voltage, ofa voltage applied to a gate of a SGS transistor of the one memory stringbefore initiating ramp up, to the erase voltage, of a voltage applied toa source line connected to a source-side end of the memory string.

For erase-inhibiting on memory strings, VERA bias needs to be completelycutoff on unselected memory strings. To achieve this, embodimentsdescribed herein provide these improvements over a conventional erasesequence including ramping up to cut off SGD of unselected memorystrings during the erase operation, and offset of ramp up timing on SGSand SGD before holes that are generated by GIDL run into the memorystrings. Shifting of ramp up timing on SGS and SGD on unselected memorystrings can help to cut-off VERA bias. Further, embodiments describedherein provide for a well-controlled erase state, which allows for lessdisturb induced by neighbor word-lines and less degradation of dataretention. In addition, embodiments described herein decrease thepossibility of hitting erase saturation on a memory cell, which improvesmemory cell reliabilities.

A programming operation for a set of memory cells of a memory devicetypically involves applying a series of program voltages to the memorycells after the memory cells are provided in an erased state. Eachprogram voltage is provided in a program loop, also referred to as aprogram-verify iteration. For example, the program voltage may beapplied to a word line which is connected to control gates of the memorycells. In one approach, incremental step pulse programming is performed,where the program voltage is increased by a step size in each programloop. Verify operations may be performed after each program voltage todetermine whether the memory cells have completed programming. Whenprogramming is completed for a memory cell, it can be locked out fromfurther programming while programming continues for other memory cellsin subsequent program loops.

Each memory cell may be associated with a data state according to writedata in a program command. Based on its data state, a memory cell willeither remain in the erased state or be programmed to a data state (aprogrammed data state) different from the erased state. For example, ina one-bit per cell memory device (single-level cell (SLC)), there aretwo data states including the erased state and one higher data state. Ina two-bit per cell memory device (multi-level cell (MLC)), there arefour data states including the erased state and three higher data statesreferred to as the A, B and C data states (see FIG. 9 ). In a three-bitper cell memory device (triple-level cell (TLC)), there are eight datastates including the erased state and seven higher data states referredto as the A, B, C, D, E, F and G data states (see FIG. 10 ). In afour-bit per cell memory device (quad-level cell (QLC)), there aresixteen data states including the erased state and fifteen higher datastates referred to as the Er, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, Eand F data states (see FIG. 11 ). Each memory cell may store a datastate (e.g., a binary value) and is programmed to a threshold voltagestate corresponding to the data state. Each state represents a differentvalue and is assigned a voltage window including a range of possiblethreshold voltages.

When a program command is issued, the write data is stored in latchesassociated with the memory cells. During programming, the latches of amemory cell can be read to determine the data state to which the cell isto be programmed. Each programmed data state is associated with a verifyvoltage such that a memory cell with a given data state is considered tohave completed programming when a sensing operation determines itsthreshold voltage (Vth) is above the associated verify voltage. Asensing operation can determine whether a memory cell has a Vth abovethe associated verify voltage by applying the associated verify voltageto the control gate and sensing a current through the memory cell. Ifthe current is relatively high, this indicates the memory cell is in aconductive state, such that the Vth is less than the control gatevoltage. If the current is relatively low, this indicates the memorycell is in a non-conductive state, such that the Vth is above thecontrol gate voltage.

The verify voltage which is used to determine that a memory cell hascompleted programming may be referred to as a final or lockout verifyvoltage. In some cases, an additional verify voltage may be used todetermine that a memory cell is close to completion of the programming.This additional verify voltage may be referred to as an offset verifyvoltage, and may be lower than the final verify voltage. When a memorycell is close to completion of programming, the programming speed of thememory cell may be reduced such as by elevating a voltage of arespective bit line during one or more subsequent program voltages. Forexample, in FIG. 9 , a memory cell which is to be programmed to the Adata state may be subject to verify tests at VvAL, an offset verifyvoltage of the A data state, and VvA, a final verify voltage of the Adata state.

To help further illustrate the foregoing, FIG. 1A will now be described.FIG. 1A is a block diagram of an example memory device. The memorydevice 100 may include one or more memory die 108. The memory die 108includes a memory structure 126 of memory cells, such as an array ofmemory cells, control circuitry 110, and read/write circuits 128. Thememory structure 126 is addressable by word lines via a row decoder 124and by bit lines via a column decoder 132. The read/write circuits 128include multiple sense blocks SB1, SB2, . . . , SBp (sensing circuitry)and allow a page of memory cells to be read or programmed in parallel.Typically a controller 122 is included in the same memory device 100(e.g., a removable storage card) as the one or more memory die 108.Commands and data are transferred between the host 140 and controller122 via a data bus 120, and between the controller and the one or morememory die 108 via lines 118.

The memory structure can be 2D or 3D. The memory structure may compriseone or more array of memory cells including a 3D array. The memorystructure may comprise a monolithic three dimensional memory structurein which multiple memory levels are formed above (and not in) a singlesubstrate, such as a wafer, with no intervening substrates. The memorystructure may comprise any type of non-volatile memory that ismonolithically formed in one or more physical levels of arrays of memorycells having an active area disposed above a silicon substrate. Thememory structure may be in a non-volatile memory device having circuitryassociated with the operation of the memory cells, whether theassociated circuitry is above or within the substrate.

The control circuitry 110 cooperates with the read/write circuits 128 toperform memory operations on the memory structure 126, and includes astate machine 112, an on-chip address decoder 114, and a power controlmodule 116. The state machine 112 provides chip-level control of memoryoperations. A storage region 113 may be provided, e.g., for verifyparameters as described herein.

The on-chip address decoder 114 provides an address interface betweenthat used by the host or a memory controller to the hardware addressused by the decoders 124 and 132. The power control module 116 controlsthe power and voltages supplied to the word lines and bit lines duringmemory operations. It can include drivers for word lines, SGS and SGDtransistors and source lines. The sense blocks can include bit linedrivers, in one approach. An SGS transistor is a select gate transistorat a source end of a NAND string, and an SGD transistor is a select gatetransistor at a drain end of a NAND string.

In some implementations, some of the components can be combined. Invarious designs, one or more of the components (alone or incombination), other than memory structure 126, can be thought of as atleast one control circuit which is configured to perform the actionsdescribed herein. For example, a control circuit may include any one of,or a combination of, control circuitry 110, state machine 112, decoders114/132, power control module 116, sense blocks SBb, SB2, . . . , SBp,read/write circuits 128, controller 122, and so forth.

The control circuits can include a programming circuit configured toprogram memory cells of a word line of a block and verify the set of thememory cells. The control circuits can also include a counting circuitconfigured to determine a number of memory cells that are verified to bein a data state. The control circuits can also include a determinationcircuit configured to determine, based on the number, whether the blockis faulty.

For example, FIG. 1B is a block diagram of an example control circuit150 which comprises a programming circuit 151, a counting circuit 152and a determination circuit 153. The programming circuit may includesoftware, firmware and/or hardware which implements, e.g., steps 1202and 1204 of FIG. 12 . The counting circuit may include software,firmware and/or hardware which implements, e.g., step 1206 of FIG. 12 .The determination circuit may include software, firmware and/or hardwarewhich implements, e.g., step 1208 of FIG. 12 .

The off-chip controller 122 may comprise a processor 122 c, storagedevices (memory) such as ROM 122 a and RAM 122 b and an error-correctioncode (ECC) engine 245. The ECC engine can correct a number of readerrors which are caused when the upper tail of a Vth distributionbecomes too high. However, uncorrectable errors may exist in some cases.The techniques provided herein reduce the likelihood of uncorrectableerrors.

The storage device comprises code such as a set of instructions, and theprocessor is operable to execute the set of instructions to provide thefunctionality described herein. Alternatively or additionally, theprocessor can access code from a storage device 126 a of the memorystructure, such as a reserved area of memory cells in one or more wordlines.

For example, code can be used by the controller 122 to access the memorystructure such as for programming, read and erase operations. The codecan include boot code and control code (e.g., set of instructions). Theboot code is software that initializes the controller during a bootingor startup process and enables the controller to access the memorystructure. The code can be used by the controller to control one or morememory structures. Upon being powered up, the processor 122 c fetchesthe boot code from the ROM 122 a or storage device 126 a for execution,and the boot code initializes the system components and loads thecontrol code into the RAM 122 b. Once the control code is loaded intothe RAM, it is executed by the processor. The control code includesdrivers to perform basic tasks such as controlling and allocatingmemory, prioritizing the processing of instructions, and controllinginput and output ports.

In one embodiment, the host is a computing device (e.g., laptop,desktop, smartphone, tablet, digital camera) that includes one or moreprocessors, one or more processor readable storage devices (RAM, ROM,flash memory, hard disk drive, solid state memory) that store processorreadable code (e.g., software) for programming the one or moreprocessors to perform the methods described herein. The host may alsoinclude additional system memory, one or more input/output interfacesand/or one or more input/output devices in communication with the one ormore processors.

Other types of non-volatile memory in addition to NAND flash memory canalso be used.

Semiconductor memory devices include volatile memory devices, such asdynamic random access memory (“DRAM”) or static random access memory(“SRAM”) devices, non-volatile memory devices, such as resistive randomaccess memory (“ReRAM”), electrically erasable programmable read onlymemory (“EEPROM”), flash memory (which can also be considered a subsetof EEPROM), ferroelectric random access memory (“FRAM”), andmagnetoresistive random access memory (“MRAM”), and other semiconductorelements capable of storing information. Each type of memory device mayhave different configurations. For example, flash memory devices may beconfigured in a NAND or a NOR configuration.

The memory devices can be formed from passive and/or active elements, inany combinations. By way of non-limiting example, passive semiconductormemory elements include ReRAM device elements, which in some embodimentsinclude a resistivity switching storage element, such as an anti-fuse orphase change material, and optionally a steering element, such as adiode or transistor. Further by way of non-limiting example, activesemiconductor memory elements include EEPROM and flash memory deviceelements, which in some embodiments include elements containing a chargestorage region, such as a floating gate, conductive nanoparticles, or acharge storage dielectric material.

Multiple memory elements may be configured so that they are connected inseries or so that each element is individually accessible. By way ofnon-limiting example, flash memory devices in a NAND configuration (NANDmemory) typically contain memory elements connected in series. A NANDstring is an example of a set of series-connected transistors comprisingmemory cells and SG transistors.

A NAND memory array may be configured so that the array is composed ofmultiple strings of memory in which a string is composed of multiplememory elements sharing a single bit line and accessed as a group.Alternatively, memory elements may be configured so that each element isindividually accessible, e.g., a NOR memory array. NAND and NOR memoryconfigurations are examples, and memory elements may be otherwiseconfigured.

The semiconductor memory elements located within and/or over a substratemay be arranged in two or three dimensions, such as a two dimensionalmemory structure or a three dimensional memory structure.

In a two dimensional memory structure, the semiconductor memory elementsare arranged in a single plane or a single memory device level.Typically, in a two dimensional memory structure, memory elements arearranged in a plane (e.g., in an x-y direction plane) which extendssubstantially parallel to a major surface of a substrate that supportsthe memory elements. The substrate may be a wafer over or in which thelayer of the memory elements are formed or it may be a carrier substratewhich is attached to the memory elements after they are formed. As anon-limiting example, the substrate may include a semiconductor such assilicon.

The memory elements may be arranged in the single memory device level inan ordered array, such as in a plurality of rows and/or columns.However, the memory elements may be arrayed in non-regular ornon-orthogonal configurations. The memory elements may each have two ormore electrodes or contact lines, such as bit lines and word lines.

A three dimensional memory array is arranged so that memory elementsoccupy multiple planes or multiple memory device levels, thereby forminga structure in three dimensions (i.e., in the x, y and z directions,where the z direction is substantially perpendicular and the x and ydirections are substantially parallel to the major surface of thesubstrate).

As a non-limiting example, a three dimensional memory structure may bevertically arranged as a stack of multiple two dimensional memory devicelevels. As another non-limiting example, a three dimensional memoryarray may be arranged as multiple vertical columns (e.g., columnsextending substantially perpendicular to the major surface of thesubstrate, i.e., in the y direction) with each column having multiplememory elements. The columns may be arranged in a two dimensionalconfiguration, e.g., in an x-y plane, resulting in a three dimensionalarrangement of memory elements with elements on multiple verticallystacked memory planes. Other configurations of memory elements in threedimensions can also constitute a three dimensional memory array.

By way of non-limiting example, in a three dimensional NAND memoryarray, the memory elements may be coupled together to form a NAND stringwithin a single horizontal (e.g., x-y) memory device level.Alternatively, the memory elements may be coupled together to form avertical NAND string that traverses across multiple horizontal memorydevice levels. Other three dimensional configurations can be envisionedwherein some NAND strings contain memory elements in a single memorylevel while other strings contain memory elements which span throughmultiple memory levels. Three dimensional memory arrays may also bedesigned in a NOR configuration and in a ReRAM configuration.

Typically, in a monolithic three dimensional memory array, one or morememory device levels are formed above a single substrate. Optionally,the monolithic three dimensional memory array may also have one or morememory layers at least partially within the single substrate. As anon-limiting example, the substrate may include a semiconductor such assilicon. In a monolithic three dimensional array, the layersconstituting each memory device level of the array are typically formedon the layers of the underlying memory device levels of the array.However, layers of adjacent memory device levels of a monolithic threedimensional memory array may be shared or have intervening layersbetween memory device levels.

Then again, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device having multiplelayers of memory. For example, non-monolithic stacked memories can beconstructed by forming memory levels on separate substrates and thenstacking the memory levels atop each other. The substrates may bethinned or removed from the memory device levels before stacking, but asthe memory device levels are initially formed over separate substrates,the resulting memory arrays are not monolithic three dimensional memoryarrays. Further, multiple two dimensional memory arrays or threedimensional memory arrays (monolithic or non-monolithic) may be formedon separate chips and then packaged together to form a stacked-chipmemory device.

Associated circuitry is typically required for operation of the memoryelements and for communication with the memory elements. As non-limitingexamples, memory devices may have circuitry used for controlling anddriving memory elements to accomplish functions such as programming andreading. This associated circuitry may be on the same substrate as thememory elements and/or on a separate substrate. For example, acontroller for memory read-write operations may be located on a separatecontroller chip and/or on the same substrate as the memory elements.

One of skill in the art will recognize that this technology is notlimited to the two dimensional and three dimensional exemplarystructures described but covers all relevant memory structures withinthe spirit and scope of the technology as described herein and asunderstood by one of skill in the art.

FIG. 2 depicts blocks of memory cells in an example two-dimensionalconfiguration of the memory array 126 of FIG. 1 . The memory array caninclude many blocks. Each example block 200, 210 includes a number ofNAND strings and respective bit lines, e.g., BL0, BL1, . . . which areshared among the blocks. Each NAND string is connected at one end to adrain select gate (SGD), and the control gates of the drain select gatesare connected via a common SGD line. The NAND strings are connected attheir other end to a source select gate which, in turn, is connected toa common source line 220. Sixteen word lines, for example, WL0-WL15,extend between the source select gates and the drain select gates. Insome cases, dummy word lines, which contain no user data, can also beused in the memory array adjacent to the select gate transistors. Suchdummy word lines can shield the edge data word line from certain edgeeffects.

One type of non-volatile memory which may be provided in the memoryarray is a floating gate memory. See FIGS. 3A and 3B. Other types ofnon-volatile memory can also be used. For example, a charge-trappingmemory cell uses a non-conductive dielectric material in place of aconductive floating gate to store charge in a non-volatile manner. SeeFIGS. 4A and 4B. A triple layer dielectric formed of silicon oxide,silicon nitride and silicon oxide (“ONO”) is sandwiched between aconductive control gate and a surface of a semi-conductive substrateabove the memory cell channel. The cell is programmed by injectingelectrons from the cell channel into the nitride, where they are trappedand stored in a limited region. This stored charge then changes thethreshold voltage of a portion of the channel of the cell in a mannerthat is detectable. The cell is erased by injecting hot holes into thenitride. A similar cell can be provided in a split-gate configurationwhere a doped polysilicon gate extends over a portion of the memory cellchannel to form a separate select transistor.

In another approach, NROM cells are used. Two bits, for example, arestored in each NROM cell, where an ONO dielectric layer extends acrossthe channel between source and drain diffusions. The charge for one databit is localized in the dielectric layer adjacent to the drain, and thecharge for the other data bit localized in the dielectric layer adjacentto the source. Multi-state data storage is obtained by separatelyreading binary states of the spatially separated charge storage regionswithin the dielectric. Other types of non-volatile memory are alsoknown.

FIG. 3A depicts a cross-sectional view of example floating gate memorycells in NAND strings. A bit line or NAND string direction goes into thepage, and a word line direction goes from left to right. As an example,word line 324 extends across NAND strings which include respectivechannel regions 306, 316 and 326. The memory cell 300 includes a controlgate 302, a floating gate 304, a tunnel oxide layer 305 and the channelregion 306. The memory cell 310 includes a control gate 312, a floatinggate 314, a tunnel oxide layer 315 and the channel region 316. Thememory cell 320 includes a control gate 322, a floating gate 321, atunnel oxide layer 325 and the channel region 326. Each memory cell isin a different respective NAND string. An inter-poly dielectric (IPD)layer 328 is also depicted. The control gates are portions of the wordline. A cross-sectional view along line 329 is provided in FIG. 3B.

The control gate wraps around the floating gate, increasing the surfacecontact area between the control gate and floating gate. This results inhigher IPD capacitance, leading to a higher coupling ratio which makesprogramming and erase easier. However, as NAND memory devices are scaleddown, the spacing between neighboring cells becomes smaller so there isalmost no space for the control gate and the IPD between two adjacentfloating gates. As an alternative, as shown in FIGS. 4A and 4B, the flator planar memory cell has been developed in which the control gate isflat or planar; that is, it does not wrap around the floating gate, andits only contact with the charge storage layer is from above it. In thiscase, there is no advantage in having a tall floating gate. Instead, thefloating gate is made much thinner. Further, the floating gate can beused to store charge, or a thin charge trap layer can be used to trapcharge. This approach can avoid the issue of ballistic electrontransport, where an electron can travel through the floating gate aftertunneling through the tunnel oxide during programming.

FIG. 3B depicts a cross-sectional view of the structure of FIG. 3A alongline 329. The NAND string 330 includes an SGS transistor 331, examplememory cells 300, 333, . . . , 334 and 335, and an SGD transistor 336.The memory cell 300, as an example of each memory cell, includes thecontrol gate 302, the IPD layer 328, the floating gate 304 and thetunnel oxide layer 305, consistent with FIG. 3A. Passageways in the IPDlayer in the SGS and SGD transistors allow the control gate layers andfloating gate layers to communicate. The control gate and floating gatelayers may be polysilicon and the tunnel oxide layer may be siliconoxide, for instance. The IPD layer can be a stack of nitrides (N) andoxides (O) such as in a N—O—N—O—N configuration.

The NAND string may be formed on a substrate which comprises a p-typesubstrate region 355, an n-type well 356 and a p-type well 357. N-typesource/drain diffusion regions sd1, sd2, sd3, sd4, sd5, sd6 and sd7 areformed in the p-type well. A channel voltage, Vch, may be applieddirectly to the channel region of the substrate.

FIG. 4A depicts a cross-sectional view of example charge-trapping memorycells in NAND strings. The view is in a word line direction of memorycells comprising a flat control gate and charge-trapping regions as a 2Dexample of memory cells in the memory cell array 126 of FIG. 1 .Charge-trapping memory can be used in NOR and NAND flash memory device.This technology uses an insulator such as a SiN film to store electrons,in contrast to a floating-gate MOSFET technology which uses a conductorsuch as doped polycrystalline silicon to store electrons. As an example,a word line (WL) 424 extends across NAND strings which includerespective channel regions 406, 416 and 426. Portions of the word lineprovide control gates 402, 412 and 422. Below the word line is an IPDlayer 428, charge-trapping layers 404, 414 and 421, polysilicon layers405, 415 and 425 and tunneling layer layers 409, 407 and 408. Eachcharge-trapping layer extends continuously in a respective NAND string.

A memory cell 400 includes the control gate 402, the charge-trappinglayer 404, the polysilicon layer 405 and a portion of the channel region406. A memory cell 410 includes the control gate 412, thecharge-trapping layer 414, a polysilicon layer 415 and a portion of thechannel region 416. A memory cell 420 includes the control gate 422, thecharge-trapping layer 421, the polysilicon layer 425 and a portion ofthe channel region 426.

A flat control gate is used here instead of a control gate that wrapsaround a floating gate. One advantage is that the charge-trapping layercan be made thinner than a floating gate. Additionally, the memory cellscan be placed closer together.

FIG. 4B depicts a cross-sectional view of the structure of FIG. 4A alongline 429. The view shows a NAND string 430 having a flat control gateand a charge-trapping layer. The NAND string 430 includes an SGStransistor 431, example memory cells 400, 433, . . . , 434 and 435, andan SGD transistor 436.

The NAND string may be formed on a substrate which comprises a p-typesubstrate region 455, an n-type well 456 and a p-type well 457. N-typesource/drain diffusion regions sd1, sd2, sd3, sd4, sd5, sd6 and sd7 areformed in the p-type well 457. A channel voltage, Vch, may be applieddirectly to the channel region of the substrate. The memory cell 400includes the control gate 402 and the IPD layer 428 above thecharge-trapping layer 404, the polysilicon layer 405, the tunnelinglayer 409 and the channel region 406.

The control gate layer may be polysilicon and the tunneling layer may besilicon oxide, for instance. The IPD layer can be a stack of high-kdielectrics such as AlOx or HfOx which help increase the coupling ratiobetween the control gate layer and the charge-trapping or charge storinglayer. The charge-trapping layer can be a mix of silicon nitride andoxide, for instance.

The SGD and SGS transistors have the same configuration as the memorycells but with a longer channel length to ensure that current is cutoffin an inhibited NAND string.

In this example, the layers 404, 405 and 409 extend continuously in theNAND string. In another approach, portions of the layers 404, 405 and409 which are between the control gates 402, 412 and 422 can be removed,exposing a top surface of the channel 406.

FIG. 5A depicts an example block diagram of the sense block SB1 of FIG.1 . In one approach, a sense block comprises multiple sense circuits.Each sense circuit is associated with data latches. For example, theexample sense circuits 550 a, 551 a, 552 a and 553 a are associated withthe data latches 550 b, 551 b, 552 b and 553 b, respectively. In oneapproach, different subsets of bit lines can be sensed using differentrespective sense blocks. This allows the processing load which isassociated with the sense circuits to be divided up and handled by arespective processor in each sense block. For example, a sense circuitcontroller 560 in SB1 can communicate with the set of sense circuits andlatches. The sense circuit controller may include a pre-charge circuit561 which provides a voltage to each sense circuit for setting apre-charge voltage. In one possible approach, the voltage is provided toeach sense circuit independently, e.g., via the data bus, DBUS 503 and alocal bus such as LBUS1 or LBUS2 in FIG. 5B. In another possibleapproach, a common voltage is provided to each sense circuitconcurrently, e.g., via the line 505 in FIG. 5B. The sense circuitcontroller may also include a memory 562 and a processor 563. Asmentioned also in connection with FIG. 2 , the memory 562 may store codewhich is executable by the processor to perform the functions describedherein. These functions can include reading latches which are associatedwith the sense circuits, setting bit values in the latches and providingvoltages for setting pre-charge levels in sense nodes of the sensecircuits. Further example details of the sense circuit controller andthe sense circuits 550 a and 551 a are provided below.

FIG. 5B depicts another example block diagram of the sense block SB1 ofFIG. 1 . The sense circuit controller 560 communicates with multiplesense circuits including example sense circuits 550 a and 551 a, alsoshown in FIG. 5A. The sense circuit 550 a includes latches 550 b,including a trip latch 526, an offset verify latch 527 and data statelatches 528. The sense circuit further includes a voltage clamp 521 suchas a transistor which sets a pre-charge voltage at a sense node 522. Asense node to bit line (BL) switch 523 selectively allows the sense nodeto communicate with a bit line 525, e.g., the sense node is electricallyconnected to the bit line so that the sense node voltage can decay. Thebit line 525 is connected to one or more memory cells such as a memorycell MC1. A voltage clamp 524 can set a voltage on the bit line, such asduring a sensing operation or during a program voltage. A local bus,LBUS1, allows the sense circuit controller to communicate withcomponents in the sense circuit, such as the latches 550 b and thevoltage clamp in some cases. To communicate with the sense circuit 550a, the sense circuit controller provides a voltage via a line 502 to atransistor 504 to connect LBUS1 with a data bus DBUS, 503. Thecommunicating can include sending data to the sense circuit and/orreceive data from the sense circuit.

The sense circuit controller can communicate with different sensecircuits in a time-multiplexed manner, for instance. A line 505 may beconnected to the voltage clamp in each sense circuit, in one approach.

The sense circuit 551 a includes latches 551 b, including a trip latch546, an offset verify latch 547 and data state latches 548. A voltageclamp 541 may be used to set a pre-charge voltage at a sense node 542. Asense node to bit line (BL) switch 543 selectively allows the sense nodeto communicate with a bit line 545, and a voltage clamp 544 can set avoltage on the bit line. The bit line 545 is connected to one or morememory cells such as a memory cell MC2. A local bus, LBUS2, allows thesense circuit controller to communicate with components in the sensecircuit, such as the latches 551 b and the voltage clamp in some cases.To communicate with the sense circuit 551 a, the sense circuitcontroller provides a voltage via a line 501 to a transistor 506 toconnect LBUS2 with DBUS.

The sense circuit 550 a may be a first sense circuit which comprises afirst trip latch 526 and the sense circuit 551 a may be a second sensecircuit which comprises a second trip latch 546.

The sense circuit 550 a is an example of a first sense circuitcomprising a first sense node 522, where the first sense circuit isassociated with a first memory cell MC1 and a first bit line 525. Thesense circuit 551 a is an example of a second sense circuit comprising asecond sense node 542, where the second sense circuit is associated witha second memory cell MC2 and a second bit line 545.

FIG. 6A is a perspective view of a set of blocks 600 in an examplethree-dimensional configuration of the memory array 126 of FIG. 1 . Onthe substrate are example blocks BLK0, BLK1, BLK2 and BLK3 of memorycells (storage elements) and a peripheral area 604 with circuitry foruse by the blocks. For example, the circuitry can include voltagedrivers 605 which can be connected to control gate layers of the blocks.In one approach, control gate layers at a common height in the blocksare commonly driven. The substrate 601 can also carry circuitry underthe blocks, along with one or more lower metal layers which arepatterned in conductive paths to carry signals of the circuitry. Theblocks are formed in an intermediate region 602 of the memory device. Inan upper region 603 of the memory device, one or more upper metal layersare patterned in conductive paths to carry signals of the circuitry.Each block comprises a stacked area of memory cells, where alternatinglevels of the stack represent word lines. In one possible approach, eachblock has opposing tiered sides from which vertical contacts extendupward to an upper metal layer to form connections to conductive paths.While four blocks are depicted as an example, two or more blocks can beused, extending in the x- and/or y-directions.

In one possible approach, the length of the plane, in the x-direction,represents a direction in which signal paths to word lines extend in theone or more upper metal layers (a word line or SGD line direction), andthe width of the plane, in the y-direction, represents a direction inwhich signal paths to bit lines extend in the one or more upper metallayers (a bit line direction). The z-direction represents a height ofthe memory device.

FIG. 6B depicts an example cross-sectional view of a portion of one ofthe blocks of FIG. 6A. The block comprises a stack 610 of alternatingconductive and dielectric layers. In this example, the conductive layerscomprise two SGD layers, two SGS layers and four dummy word line layersDWLD0, DWLD1, DWLS0 and DWLS1, in addition to data word line layers(word lines) WLL0-WLL10. The dielectric layers are labelled as DL0-DL19.Further, regions of the stack which comprise NAND strings NS1 and NS2are depicted. Each NAND string encompasses a memory hole 618 or 619which is filled with materials which form memory cells adjacent to theword lines. A region 622 of the stack is shown in greater detail in FIG.6D.

The stack includes a substrate 611, an insulating film 612 on thesubstrate, and a portion of a source line SL. NS1 has a source-end 613at a bottom 614 of the stack and a drain-end 615 at a top 616 of thestack. Metal-filled slits 617 and 620 may be provided periodicallyacross the stack as interconnects which extend through the stack, suchas to connect the source line to a line above the stack. The slits maybe used during the formation of the word lines and subsequently filledwith metal. A portion of a bit line BL0 is also depicted. A conductivevia 621 connects the drain-end 615 to BL0.

FIG. 6C depicts a plot of memory hole diameter in the stack of FIG. 6B.The vertical axis is aligned with the stack of FIG. 6B and depicts awidth (wMH), e.g., diameter, of the memory holes 618 and 619. The wordline layers WLL0-WLL10 of FIG. 6A are repeated as an example and are atrespective heights z0-z10 in the stack. In such a memory device, thememory holes which are etched through the stack have a very high aspectratio. For example, a depth-to-diameter ratio of about 25-30 is common.The memory holes may have a circular cross-section. Due to the etchingprocess, the memory hole width can vary along the length of the hole.Typically, the diameter becomes progressively smaller from the top tothe bottom of the memory hole. That is, the memory holes are tapered,narrowing at the bottom of the stack. In some cases, a slight narrowingoccurs at the top of the hole near the select gate so that the diameterbecomes slight wider before becoming progressively smaller from the topto the bottom of the memory hole.

Due to the non-uniformity in the width of the memory hole, theprogramming speed, including the program slope and erase speed of thememory cells can vary based on their position along the memory hole,e.g., based on their height in the stack. With a smaller diameter memoryhole, the electric field across the tunnel oxide is relatively stronger,so that the programming and erase speed is relatively higher. Oneapproach is to define groups of adjacent word lines for which the memoryhole diameter is similar, e.g., within a defined range of diameter, andto apply an optimized verify scheme for each word line in a group.Different groups can have different optimized verify schemes.

FIG. 6D depicts a close-up view of the region 622 of the stack of FIG.6B. Memory cells are formed at the different levels of the stack at theintersection of a word line layer and a memory hole. In this example,SGD transistors 680 and 681 are provided above dummy memory cells 682and 683 and a data memory cell MC. A number of layers can be depositedalong the sidewall (SW) of the memory hole 630 and/or within each wordline layer, e.g., using atomic layer deposition. For example, eachcolumn (e.g., the pillar which is formed by the materials within amemory hole) can include a charge-trapping layer or film 663 such as SiNor other nitride, a tunneling layer 664, a polysilicon body or channel665, and a dielectric core 666. A word line layer can include a blockingoxide/block high-k material 660, a metal barrier 661, and a conductivemetal 662 such as Tungsten as a control gate. For example, control gates690, 691, 692, 693 and 694 are provided. In this example, all of thelayers except the metal are provided in the memory hole. In otherapproaches, some of the layers can be in the control gate layer.Additional pillars are similarly formed in the different memory holes. Apillar can form a columnar active area (AA) of a NAND string.

When a memory cell is programmed, electrons are stored in a portion ofthe charge-trapping layer which is associated with the memory cell.These electrons are drawn into the charge-trapping layer from thechannel, and through the tunneling layer. The Vth of a memory cell isincreased in proportion to the amount of stored charge. During an eraseoperation, the electrons return to the channel.

Each of the memory holes can be filled with a plurality of annularlayers comprising a blocking oxide layer, a charge trapping layer, atunneling layer and a channel layer. A core region of each of the memoryholes is filled with a body material, and the plurality of annularlayers are between the core region and the word line in each of thememory holes.

The NAND string can be considered to have a floating body channelbecause the length of the channel is not formed on a substrate. Further,the NAND string is provided by a plurality of word line layers above oneanother in a stack, and separated from one another by dielectric layers.

FIG. 7A depicts a top view of an example word line layer WLL0 of thestack of FIG. 6B. As mentioned, a 3D memory device can comprise a stackof alternating conductive and dielectric layers. The conductive layersprovide the control gates of the SG transistors and memory cells. Thelayers used for the SG transistors are SG layers and the layers used forthe memory cells are word line layers. Further, memory holes are formedin the stack and filled with a charge-trapping material and a channelmaterial. As a result, a vertical NAND string is formed. Source linesare connected to the NAND strings below the stack and bit lines areconnected to the NAND strings above the stack.

A block BLK in a 3D memory device can be divided into sub-blocks, whereeach sub-block comprises a set of NAND string which have a common SGDcontrol line. For example, see the SGD lines/control gates SGD0, SGD1,SGD2 and SGD3 in the sub-blocks SBa, SBb, SBc and SBd, respectively. Thesub-blocks SBa, SBb, SBc and SBd may also be referred herein as a stringof memory cells of a word line. As described, a string of memory cellsof a word line may include a plurality of memory cells that are part ofthe same sub-block, and that are also disposed in the same word linelayer and/or that are configured to have their control gates biased bythe same word line and/or with the same word line voltage.

Further, a word line layer in a block can be divided into regions. Eachregion is in a respective sub-block are can extend between slits whichare formed periodically in the stack to process the word line layersduring the fabrication process of the memory device. This processing caninclude replacing a sacrificial material of the word line layers withmetal. Generally, the distance between slits should be relatively smallto account for a limit in the distance that an etchant can travellaterally to remove the sacrificial material, and that the metal cantravel to fill a void which is created by the removal of the sacrificialmaterial. For example, the distance between slits may allow for a fewrows of memory holes between adjacent slits. The layout of the memoryholes and slits should also account for a limit in the number of bitlines which can extend across the region while each bit line isconnected to a different memory cell. After processing the word linelayers, the slits can optionally be filed with metal to provide aninterconnect through the stack.

This figure and other are not necessarily to scale. In practice, theregions can be much longer in the x-direction relative to they-direction than is depicted to accommodate additional memory holes.

In this example, there are four rows of memory holes between adjacentslits. A row here is a group of memory holes which are aligned in thex-direction. Moreover, the rows of memory holes are in a staggeredpattern to increase the density of the memory holes. The word line layeror word line is divided into regions WLL0 a, WLL0 b, WLL0 c and WLL0 dwhich are each connected by a connector 713. The last region of a wordline layer in a block can be connected to a first region of a word linelayer in a next block, in one approach. The connector, in turn, isconnected to a voltage driver for the word line layer. The region WLL0 ahas example memory holes 710 and 711 along a line 712. The region WLL0 bhas example memory holes 714 and 715. The region WLL0 c has examplememory holes 716 and 717. The region WLL0 d has example memory holes 718and 719. The memory holes are also shown in FIG. 7B. Each memory holecan be part of a respective NAND string. For example, the memory holes710, 714, 716 and 718 can be part of NAND strings NS0_SBa, NS0_SBb,NS0_SBc and NS0_SBd, respectively.

Each circle represents the cross-section of a memory hole at a word linelayer or SG layer. Example circles shown with dashed lines representmemory cells which are provided by the materials in the memory hole andby the adjacent word line layer. For example, memory cells 720 and 721are in WLL0 a, memory cells 724 and 725 are in WLL0 b, memory cells 726and 727 are in WLL0 c, and memory cells 728 and 729 are in WLL0 d. Thesememory cells are at a common height in the stack.

Metal-filled slits 701, 702, 703 and 704 (e.g., metal interconnects) maybe located between and adjacent to the edges of the regions WLL0 a-WLL0d. The metal-filled slits provide a conductive path from the bottom ofthe stack to the top of the stack. For example, a source line at thebottom of the stack may be connected to a conductive line above thestack, where the conductive line is connected to a voltage driver in aperipheral region of the memory device. See also FIG. 8A for furtherdetails of the sub-blocks SBa-SBd of FIG. 7A.

FIG. 7B depicts a top view of an example top dielectric layer DL19 ofthe stack of FIG. 6B. The dielectric layer is divided into regions DL19a, DL19 b, DL19 c and DL19 d. Each region can be connected to arespective voltage driver. This allows a set of memory cells in oneregion of a word line layer to be programmed concurrently, with eachmemory cell being in a respective NAND string which is connected to arespective bit line. A voltage can be set on each bit line to allow orinhibit programming during each program voltage.

The region DL19 a has the example memory holes 710 and 711 along a line712 a which is coincident with a bit line BL0. A number of bit linesextend above the memory holes and are connected to the memory holes asindicated by the “X” symbols. BL0 is connected to a set of memory holeswhich includes the memory holes 711, 715, 717 and 719. Another examplebit line BL1 is connected to a set of memory holes which includes thememory holes 710, 714, 716 and 718. The metal-filled slits 701, 702, 703and 704 from FIG. 7A are also depicted, as they extend verticallythrough the stack. The bit lines can be numbered in a sequence BL0-BL23across the DL19 layer in the −x direction.

Different subsets of bit lines are connected to cells in different rows.For example, BL0, BL4, BL8, BL12, BL16 and BL20 are connected to cellsin a first row of cells at the right hand edge of each region. BL2, BL6,BL10, BL14, BL18 and BL22 are connected to cells in an adjacent row ofcells, adjacent to the first row at the right hand edge. BL3, BL7, BL11,BL15, BL19 and BL23 are connected to cells in a first row of cells atthe left hand edge of each region. BL1, BL5, BL9, BL13, BL17 and BL21are connected to cells in an adjacent row of cells, adjacent to thefirst row at the left hand edge.

FIG. 8A depicts example NAND strings in the sub-blocks SBa-SBd of FIG.7A. The sub-blocks are consistent with the structure of FIG. 6B. Theconductive layers in the stack are depicted for reference at the lefthand side. Each sub-block includes multiple NAND strings, where oneexample NAND string is depicted. For example, SBa comprises an exampleNAND string NS0_SBa, SBb comprises an example NAND string NS0_SBb, SBccomprises an example NAND string NS0_SBc, and SBd comprises an exampleNAND string NS0_SBd.

Additionally, NS0_SBa include SGS transistors 800 and 801, dummy memorycells 802 and 803, data memory cells 804, 805, 806, 807, 808, 809, 810,811, 812, 813 and 814, dummy memory cells 815 and 816, and SGDtransistors 817 and 818.

NS0_SBb include SGS transistors 820 and 821, dummy memory cells 822 and823, data memory cells 824, 825, 826, 827, 828, 829, 830, 831, 832, 833and 834, dummy memory cells 835 and 836, and SGD transistors 837 and838.

NS0_SBc include SGS transistors 840 and 841, dummy memory cells 842 and843, data memory cells 844, 845, 846, 847, 848, 849, 850, 851, 852, 853and 854, dummy memory cells 855 and 856, and SGD transistors 857 and858.

NS0_SBd include SGS transistors 860 and 861, dummy memory cells 862 and863, data memory cells 864, 865, 866, 867, 868, 869, 870, 871, 872, 873and 874, dummy memory cells 875 and 876, and SGD transistors 877 and878.

At a given height in the block, a set of memory cells in each sub-blockare at a common height. For example, one set of memory cells (includingthe memory cell 804) is among a plurality of memory cells formed alongtapered memory holes in a stack of alternating conductive and dielectriclayers. The one set of memory cells is at a particular height z0 in thestack. Another set of memory cells (including the memory cell 824)connected to the one word line (WLL0) are also at the particular height.In another approach, the set of memory cells (e.g., including the memorycell 812) connected to another word line (e.g., WLL8) are at anotherheight (z8) in the stack.

FIG. 8B depicts another example view of NAND strings in sub-blocks. TheNAND strings includes NS0_SBa, NS0_SBb, NS0_SBc and NS0_SBd, which have48 word lines, WL0-WL47, in this example. Each sub-block comprises a setof NAND strings which extend in the x direction and which have a commonSGD line, e.g., SGD0, SGD1, SGD2 or SGD3. In this simplified example,there is only one SGD transistor and one SGS transistor in each NANDstring. The NAND strings NS0_SBa, NS0_SBb, NS0_SBc and NS0_SBd are insub-blocks SBa, SBb, SBc and SBd, respectively. Further, example, groupsof word lines G0, G1 and G2 are depicted.

FIG. 8C generally illustrates a schematic view of three versions ofstaggered string architecture 101, 103, 105 for BiCS memory, e.g., NAND.With reference the string architecture 101, the strings are shown inrows 107-0 through 107-7 in architecture 101. Each row is shown withfour ends to the strings. A string may be connected to an adjacentstring at an end (not visible beneath this view). A first group of rows107-0 through 107-3 are shown on a left side of a dummy row 108. Asecond group of rows 107-4 through 107-7 are shown on a right side ofthe dummy row 108. The dummy row 108 separates the two groups of rows inthe staggered eight rows. A source line 109 is positioned at an edge ofthe first group and is remote from the dummy row 108. A source line 110is positioned at an edge of the second group and is remote from thedummy row 108 and source line 109.

The staggered string architectures 103, 105 for BiCS memory are similarto that of architecture 101 except additional groups are added.Architecture 103 is double the size of architecture 101 and includessixteen rows of strings with each group of four rows separated by adummy row. Architecture 105 is larger than both the architecture 101 andthe architecture 103. Architecture 105 includes twenty rows of stringswith each group of four rows separated by a dummy row 108.

These architectures 101, 103, 105 can include a chip under arraystructure, e.g., the control circuitry is under the memory array thatcan include the groups of memory strings. With the chip under arraystructure, the strings may include a direct strap contact for the sourceline for read and erase operations.

FIG. 12 depicts a waveform of an example programming operation. Thehorizontal axis depicts a program loop number and the vertical axisdepicts control gate or word line voltage. Generally, a programmingoperation can involve applying a pulse train to a selected word line,where the pulse train includes multiple program loops or program-verify(PV) iterations. The program portion of the program-verify iterationcomprises a program voltage, and the verify portion of theprogram-verify iteration comprises one or more verify voltages.

For each program voltage, a square waveform is depicted for simplicity,although other shapes are possible such as a multilevel shape or aramped shape. Further, Incremental Step Pulse Programming (ISPP) is usedin this example, in which the program voltage steps up in eachsuccessive program loop. This example uses ISPP in a single programmingstage in which the programming is completed. ISPP can also be used ineach programming stage of a multi-stage operation.

A pulse train typically includes program voltages which increasestepwise in amplitude in each program-verify iteration using a fixed ofvarying step size. A new pulse train can be applied in each programmingstage of a multi-stage programming operation, starting at an initialVpgm level and ending at a final Vpgm level which does not exceed amaximum allowed level. The initial Vpgm levels can be the same ordifferent in different programming stages. The final Vpgm levels canalso be the same or different in different programming stages. The stepsize can be the same or different in the different programming stages.In some cases, a smaller step size is used in a final programming stageto reduce Vth distribution widths.

The pulse train 900 includes a series of program voltages 901, 902, 903,904, 905, 906, 907, 908, 909, 910, 911, 912, 913, 914 and 915 that areapplied to a word line selected for programming, and an associated setof non-volatile memory cells. One, two or three verify voltages areprovided after each program voltage as an example, based on the targetdata states which are being verified. 0 V may be applied to the selectedword line between the program and verify voltages. For example, anA-state verify voltage of VvA (e.g., waveform or programming signal 916)may be applied after each of the first, second and third programvoltages 901, 902 and 903, respectively. A- and B-state verify voltagesof VvA and VvB (e.g., programming signal 917) may be applied after eachof the fourth, fifth and sixth program voltages 904, 905 and 906,respectively. A-, B- and C-state verify voltages of VvA, VvB and VvC(e.g., programming signal 918) may be applied after each of the seventhand eighth program voltages 907 and 908, respectively. B- and C-stateverify voltages of VvB and VvC (e.g., programming signal 919) may beapplied after each of the ninth, tenth and eleventh program voltages909, 910 and 911, respectively. Finally, a C-state verify voltage of VvC(e.g., programming signal 1020) may be applied after each of thetwelfth, thirteenth, fourteenth and fifteenth program voltages 912, 913,914 and 915, respectively.

FIGS. 13A and 13B show threshold voltage (Vth) distributions of memorycells in an example two-stage programming operation. Specifically, thememory cells are initially in the erased state (bits 11) as representedby the Vth distribution 1000 shown in FIG. 13A. FIG. 13B depicts Vthdistributions of memory cells after a first programming stage and asecond programming stage of the example two-stage programming operationwith four data states. While two programming stages and four data statesare shown, it should be appreciated that any number of programmingstages may be utilized (e.g., three or four programming stages) and anynumber of data states are contemplated.

In the example, the first programming stage causes the Vth of the A, Band C state cells to reach the Vth distributions 1002 a, 1004 a and 1006a, using first verify voltages of VvAf, VvBf and VvCf, respectively.This first programming stage can be a rough programming which uses arelatively large step size, for instance, so that the Vth distributions1002 a, 1004 a and 1006 a are relatively wide. The second programmingstage may use a smaller step size and causes the Vth distributions 1002a, 1004 a and 1006 a to transition to the final Vth distributions 1002,1004 and 1006 (e.g., narrower than Vth distributions 1002 a, 1004 a and1006 a), using second verify voltages of VvA, VvB, and VvC,respectively. This two-stage programming operation can achieverelatively narrow Vth distributions. A small number of A, B and C statecells (e.g., smaller than a predetermined number of the plurality ofmemory cells) may have a Vth which is below VvA, VvB or VvC,respectively, due to a bit ignore criteria.

A 3D stacked non-volatile memory device can be arranged in multipleblocks, where typically an erase operation is performed one block at atime. An erase operation can include multiple erase-verify iterationswhich are performed until an erase-verify condition is met for theblock, at which point the erase operation ends. One approach is for theerase-verify condition to allow a predetermined number of fail bits.That is, the erase operation can be declared to be successful even if asmall number of memory cells have not reached the erase state. However,this approach does not inhibit fast-erasing memory cells fromover-erase. As a result, over-erase of some of the memory cells canoccur, causing serious degradation of the memory cells as excessiveholes are accumulated in the tunneling path.

However, unlike a 2D NAND structure, where a p-well substrate is commonfor all blocks, 3D stacked non-volatile memory devices have anindividual thin poly-silicon body for each NAND string channel, whosebias can be controlled by bit line (BL), source line (SL), drain-sideselect gate (SGD) and source-side select gate (SGS) voltages. In anormal erase operation, referred to as a two-sided erase, gate-induceddrain leakage (GIDL) currents are generated at both the SGD and SGStransistors. The BL and SL are biased at Verase (VERA), and SGD and SGSare biased at Vsg. In one approach, once all the memory cells associatedwith the same bit line pass an erase-verify test (e.g., reach the erasestate), the associated bit line voltage is reduced to Vsg+(0˜2V), sothat no GIDL current is generated at the next erase pulse at the bitline/drain side. Meanwhile, the source line voltage is also reduced toVsg+(0{tilde over ( )}2V) so that, for all channels, there will be noGIDL current generated at the source line side for all the followingerase pulses of the erase operation. Thus, erase inhibit is achieved forthe memory cells which pass the erase-verify test, while those that didnot pass are then erased by GIDL current generated at the bit line sideonly, in a one-sided erase. This avoids over erase of the cells whichreach the erase state relatively quickly.

To further help avoid over erase, embodiments described herein aredirected to a string-based erase inhibit. For example, during anerase-verify iteration of an erase operation, a memory string may beinhibited or unselected once the memory string passes the erase-verifytest. In some embodiments, a memory string may be inhibited byinitiating ramp up, to an erase voltage, of a voltage applied to a gateof a SGD transistor of the memory string before initiating ramp up, tothe erase voltage, of a voltage applied to a bit line connected to adrain-side end of the memory string. In some embodiments, a memorystring may be inhibited by initiating ramp up, to the erase voltage, ofa voltage applied to a gate of a SGS transistor of the one memory stringbefore initiating ramp up, to the erase voltage, of a voltage applied toa source line connected to a source-side end of the memory string.

For erase-inhibiting on memory strings, VERA bias needs to be completelycutoff on unselected memory strings. To achieve this, embodimentsdescribed herein provide these improvements over a conventional erasesequence including ramping up to cut off SGD of unselected memorystrings during the erase operation, and offset of ramp up timing on SGSand SGD before holes that are generated by GIDL run into the memorystrings. Shifting of ramp up timing on SGS and SGD on unselected memorystrings can help to cut-off VERA bias. Further, embodiments describedherein provide for a well-controlled erase state, which allows for lessdisturb induced by neighbor word-lines and less degradation of dataretention. In addition, embodiments described herein decrease thepossibility of hitting erase saturation on a memory cell, which improvesmemory cell reliabilities.

FIG. 14A depicts threshold voltage distributions of an erased state andhigher data states. As mentioned, memory cells can be programmed so thattheir threshold voltages are in respective ranges which represent datastates. Initially, an erase operation is performed which places all ofthe memory cells in the erased state (E). Subsequently, some of thememory cells can be programmed to a higher threshold voltage such as torepresent the A, B or C data states.

The x-axis indicates a threshold voltage and the y-axis indicates anumber of storage elements. In this example, there are four data states(each represented by a threshold voltage distribution): an initialerased state 400, a soft programmed erased state (E) 402, an A state404, a B state 406 and a C state 408. Memory devices with additionaldata states, e.g., eight or sixteen data states, can also be used. Thedistribution 400 is realized after the erase operation when storageelements are typically over-erased, past the erase state 402. In theerase operation, one or more erase pulses are applied to the NAND stringat its source and/or drain ends, until the threshold voltage of thestorage elements being erased transitions below an erase-verify level,Vv-erase which can be 0 V or close to 0V, in one approach. Once theerase operation is completed for a block, the soft programming operationis performed, in which one or more positive voltage pulses are appliedto the control gates of the storage elements, such as via a word line,to increase the threshold voltages of some or all of the storageelements in the distribution 400 closer to and below a soft programming(SPGM) verify level, Vv-spgm, to the erased state 402. For example, acertain, small fraction of the storage elements may be soft programmedto have a Vth above Vv-spgm, at which point the soft programing ends,leaving most of the other storage elements with a Vth which is close to,but below, Vv-spgm. Vv-spgm is typically above or equal to Vv-erase. Thesoft programming operation advantageously results in a narrow erasestate distribution 402. Once the soft programming operation is complete,programming to higher data states can occur, such as to states A, B andC using verify levels VvA, VvB and VvC, respectively. A subsequent readoperation can use the levels VreadA, VreadB and VreadC. A seterase-verify condition can be met based on whether one or more memorycells have a Vth below Vv-erase.

FIG. 14B depicts a series 450 of erase pulses (Verase0 to Verase7) andverify pulses (see, e.g., example erase-verify pulse 472) in an eraseoperation. The erase pulses and verify pulses are presented together forunderstanding although they are applied to different portions of thememory device. An erase operation can include multiple erase-verifyiterations, e.g., EV0 to EV7. Each erase-verify iteration can include anerase portion followed by a verify portion. Examples erase portions 452,454, 456, 458, 460, 462, 464 and 466 are provided for erase-verifyiterations EV0, EV1, EV2, EV3, EV4, EV5, EV6 to EV7. Example verifyportion 472 having an amplitude of Vv-erase follows erase portion 452.In the erase portion, an erase pulse or voltage is applied to one orboth ends of a NAND string. Each erase portion can have a first portionwhich is applied in a preparation phase, and a second portion which isapplied in charge up and erase phases, as discussed further below. Forexample, erase portion 452 has a first portion 468 and a second portion470. In this example, the first portion of each erase portion has anamplitude of Vsg (an initial lower level), and the second portions ofthe erase portions have amplitudes (subsequent peak levels) of Verase0,Verase1, Verase2, Verase3, Verase4, Verase5, Verase6 and Verase7, whichincrease according to a step size of Verase-step.

The erase pulses can thus step up in amplitude in each iteration, in oneapproach. In the verify portion, a determination is made as to whetherthe Vth of a selected memory cell which is to be erased has fallen belowVv-erase. This can include determining whether the selected memory cellis in a conductive state when a word line voltage of Vv-erase is appliedto the selected memory cell. If the selected memory cell is in aconductive state, Vth<Vv-erase and the selected memory cell has beenerased. If the selected memory cell is in a non-conductive state,Vth>Vv-erase and the selected memory cell has not yet been erased.

To help further explore the above in further detail, FIG. 15 will now bedescribed. FIG. 15 depicts a method 1500 of implementing a string-basederase inhibit scheme, in accordance with embodiments described herein.In some embodiments, method 1500 may be implemented by a controller,control circuitry, a processor, and/or the like, as described elsewhereherein. As shown in FIG. 15 , method 1500 starts at step 1502. In step1502, for the plurality of memory strings, one erase-verify iteration inan erase operation is performed. The one erase-verify iteration includesdetermining whether at least one memory string of the plurality ofmemory strings passes an erase-verify test. For example, with referenceto FIGS. 1A and 1B, control circuitry 110 may perform the oneerase-verify iteration in an erase operation including determiningwhether at least one memory string of the plurality of memory stringspasses an erase-verify test.

More specifically, in some embodiments, the one erase-verify iterationmay include identifying selected memory cells to be erased in a block.For example, the identifying steps can be performed by controlcircuitry, and can involve a determination of one or more selectedblocks, memory cells, NAND strings and/or sets of NAND strings. One ormore unselected blocks, memory cells, NAND strings and/or sets of NANDstrings can similarly be identified by the control circuitry. An eraseoperation can be initiated by control circuitry of the memory deviceindependently of an external host controller, or in response to anexternal host controller, for instance.

Further, in step 1502, the value of Verase may be initialized and anerase portion of an erase-verify iteration may begin. As described, insome embodiments, the erase portion may include performing a two-sidederase of all selected NAND strings (e.g., NAND strings NS0_SBa, NS0_SBb,NS0_SBc and NS0_SBd of FIG. 8A) in a block. Generally, an eraseoperation can involve an entire block, or one or more sub-blocks.Additionally, in step 1502, the verify portion of the erase-verifyiteration is performed. For example, the verify portion may includeidentifying one or more of the selected NAND strings which pass anerase-verify test. For example, a selected NAND string may pass theerase-verify test when sensing circuitry determines that the string isconductive, when Vv-erase is applied to the control gates of theselected memory cells of the string. A selected NAND string which passesthe erase-verify test is then considered to be an unselected orinhibited NAND string.

In FIG. 15 , at step 1504, if the at least one memory string passes theerase-verify test, the at least one memory string is inhibited and anext erase-verify iteration in the erase operation is performed forremaining memory strings of the plurality of memory strings other thanthe at least one memory string. For example, with reference to FIGS. 1Aand 1B, control circuitry 110 may inhibit the at least one memory stringfor erase and perform a next erase-verify iteration in the eraseoperation for remaining memory strings of the plurality of memorystrings. For example, in some embodiments, inhibiting the memory mayinclude ramping up, to an erase voltage, of a voltage applied to a gateof a SGD transistor of the at least one memory string. Ramping up of thevoltage applied to the gate of the SGD transistor of the at least onememory string may be initiated at a time before initiating ramp up, tothe erase voltage, of a voltage applied to a bit line connected to adrain-side end of the at least one memory string. If the voltage appliedto the gate of the SGD transistor of the at least one memory string ishigher than or roughly equal to the voltage applied to a bit lineconnected to a drain-side end of the at least one memory string, thenessentially no GIDL will be generated from the SGD transistor.

Still yet, in some embodiments, inhibiting the at least one memorystring may include initiating ramp up, to the erase voltage, of avoltage applied to a gate of a SGS transistor of the at least one memorystring at a time before initiating ramp up, to the erase voltage, of avoltage applied to a source line connected to a source-side end of theat least one memory string. If the voltage applied to the gate of theSGS transistor of the at least one memory string is higher than orroughly equal to the voltage applied to a source line connected to thesource-side end of the at least one memory string, then essentially noGIDL will be generated from the SGS transistor.

To help further illustrate the foregoing, FIG. 16 will now be described.FIG. 16 an example sequence of the string-based erase inhibit schemedescribed herein. As shown in FIG. 16 , initially, an erase portion of afirst erase verify iteration 1602 is performed on memory strings S0, S1,S2, and S3. Next a verify portion of first erase-verify iteration 1602is performed. The verify portion may identify any (one or more) of theselected memory strings which pass an erase-verify test. In thisparticular scenario, memory strings S0, S1, S2, and S3 do not pass theerase-verify test. A next erase-verify iteration 1604 is performed formemory strings S0, S1, S2, and S3. In this particular scenario, memorystrings S0 and S2 do pass the erase-verify test. In a third erase-verifyiteration 1606, memory strings S0 and S2 are inhibited or unselected andthird erase-verify iteration 1606 is performed on memory strings S1 andS3. In this particular scenario, memory string S3 passes theerase-verify test. In a final erase-verify iteration 1608, memorystrings S0, S2, and S3 are inhibited or unselected and the finalerase-verify iteration is performed on only memory string S1. This helpsto prevent over-erase by not performing the erase portion oferase-verify iteration on a memory string after it has passed anerase-verify test.

To help further illustrate, FIG. 17 will now be described. FIG. 17 is anexample embodiment of a signal timing diagram of string-based eraseinhibit scheme during an erase-verify iteration in an erase operation.As shown in FIG. 17 , CELSRC ramps up from Vss to VERA at clock (CLK)cycle 11, while SGD for unselected or inhibited memory strings(SGD_USEL), SGS, and a bottom source select gate (SGSB) is ramped up ata CLK cycle before CELSRC, thereby cutting off VERA bias and preventingGIDL. In contrast, SGD for selected memory strings for erase (SGD_SEL)is ramped up at after CELSRC ramps up, thereby allowing for thegeneration of GIDL. VERA is supplied by CELSRC for source side whereGIDL is controlled by SGS and SGSB. VERA is supplied by BL for drainside where GIDL is controlled by SGD.

FIG. 18 provides an example embodiment of an expected waveform ofSGD_USEL. As depicted in FIG. 18 , SGD_USEL is applied to inhibit memorystrings. When VERA equals SGD_USEL, SGD_USEL is merged with VERA. Insome embodiments, various parameters used in implementing thestring-based erase inhibit scheme described herein may be programmedinto (e.g., programmable ROM) a particular memory device. For example, afirst parameter may enable selection of which CLK cycle (e.g., E8, E9,E10, or E11) to initiate ramp up of the voltage applied to the gate ofthe SGS transistor of the at least one memory string. Another parametermay enable selection of which CLK cycle (e.g., E8, E9, E10, or E11) toinitiate ramp up of the voltage applied to the gate of the SGDtransistor of the at least one memory string. Additionally, anotherparameter may enable selection of a voltage applied to the gate of theSGD transistor of the at least one memory string from a plurality ofinhibit voltages (e.g., 3V, 5V, 7V, or 9V).

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteachings. The described embodiments were chosen in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the claims appended hereto.

What is claimed is:
 1. A non-volatile memory device, comprising: asubstrate; a non-volatile memory cell array carried by the substrate andcomprising a plurality of memory strings, each memory string of theplurality of memory strings comprising a plurality of memory cellsarranged between a select gate drain (SGD) transistor at a drain-sideend of a memory string and a select gate source (SGS) transistor at asource-side end of the memory string; and at least one control circuitin communication with the non-volatile memory cell array, the at leastone control circuit: to perform, for the plurality of memory strings,one erase-verify iteration in an erase operation including determiningwhether at least one memory string of the plurality of memory stringspasses an erase-verify test; and if the at least one memory stringpasses the erase-verify test: to inhibit the at least one memory stringfor erase including ramping up, to an erase voltage, of a voltageapplied to a gate of a SGD transistor of the at least one memory stringat a time before initiating ramp up, to the erase voltage, of a voltageapplied to a bit line connected to a drain-side end of the at least onememory string; and to perform a next erase-verify iteration in the eraseoperation for remaining memory strings of the plurality of memorystrings other than the at least one memory string.
 2. The non-volatilememory device of claim 1, wherein to inhibit the at least one memorystring further includes initiating ramp up, to the erase voltage, of avoltage applied to a gate of a SGS transistor of the at least one memorystring at a time before initiating ramp up, to the erase voltage, of avoltage applied to a source line connected to a source-side end of theat least one memory string.
 3. The non-volatile memory device of claim2, wherein the at least one control circuit determines a clock cycle toinitiate ramp up of the voltage applied to the gate of the SGStransistor of the at least one memory string.
 4. The non-volatile memorydevice of claim 1, wherein the at least one control circuit determines aclock cycle to initiate ramp up of the voltage applied to the gate ofthe SGD transistor of the at least one memory string.
 5. Thenon-volatile memory device of claim 1, wherein the at least one controlcircuit selects the voltage applied to the gate of the SGD transistor ofthe at least one memory string from a plurality of inhibit voltages. 6.The non-volatile memory device of claim 1, wherein each memory string ofthe plurality of memory strings is a NAND string.
 7. A method ofoperating a memory apparatus comprising a plurality of memory strings,each memory string of the plurality of memory strings comprising aplurality of memory cells arranged between a select gate drain (SGD)transistor at a drain-side end of a memory string and a select gatesource (SGS) transistor at a source-side end of the memory string, themethod comprising the steps of: performing, for the plurality of memorystrings, one erase-verify iteration in an erase operation includingdetermining whether at least one memory string of the plurality ofmemory strings passes an erase-verify test; and if the at least onememory string passes the erase-verify test: inhibiting the at least onememory string for erase including ramping up, to an erase voltage, of avoltage applied to a gate of a SGD transistor of the at least one memorystring at a time before initiating ramp up, to the erase voltage, of avoltage applied to a bit line connected to a drain-side end of the atleast one memory string; and performing a next erase-verify iteration inthe erase operation for remaining memory strings of the plurality ofmemory strings other than the at least one memory string.
 8. The methodas set forth in claim 7, wherein inhibiting the at least one memorystring further includes initiating ramp up, to the erase voltage, of avoltage applied to a gate of a SGS transistor of the at least one memorystring at a time before initiating ramp up, to the erase voltage, of avoltage applied to a source line connected to a source-side end of theat least one memory string.
 9. The method as set forth in claim 8,further including the step of: determining a clock cycle to initiateramp up of the voltage applied to the gate of the SGS transistor of theat least one memory string.
 10. The method as set forth in claim 7,further including the step of: determining a clock cycle to initiateramp up of the voltage applied to the gate of the SGD transistor of theat least one memory string.
 11. The method as set forth in claim 7,further including the step of: selecting the voltage applied to the gateof the SGD transistor of the at least one memory string from a pluralityof inhibit voltages.
 12. The method as set forth in claim 7, whereineach memory string of the plurality of memory strings is a NAND string.13. An apparatus comprising: means for determining whether at least onememory string of a plurality of memory strings passes an erase-verifytest of a first erase-verify iteration in an erase operation; and meansfor ramping up, to an erase voltage, of a voltage applied to a gate of aSGD transistor of the at least one memory string if the at least onememory string passes the erase-verify test; means for performing a nexterase-verify iteration in the erase operation for remaining memorystrings of the plurality of memory strings other than the at least onememory string; and means for initiating ramp up of the voltage appliedto the gate of the SGD transistor of the at least one memory string at atime before ramp up, to the erase voltage, of a voltage applied to a bitline connected to a drain-side end of the at least one memory string.14. The apparatus of claim 13, further comprising means for initiatingramp up, to the erase voltage, of a voltage applied to a gate of a SGStransistor of the at least one memory string at a time before initiatingramp up, to the erase voltage, of a voltage applied to a source lineconnected to a source-side end of the at least one memory string. 15.The apparatus of claim 14, further comprising means for determining aclock cycle to initiate ramp up of the voltage applied to the gate ofthe SGS transistor of the at least one memory string.
 16. The apparatusof claim 13, further comprising means for determining a clock cycle toinitiate ramp up of the voltage applied to the gate of the SGDtransistor of the at least one memory string.
 17. The apparatus of claim13, further comprising means for selecting the voltage applied to thegate of the SGD transistor of the at least one memory string from aplurality of inhibit voltages.